This invention relates to fabrication techniques for the creation of new optical elements, an asymmetric or non-reciprocal diffraction grating and a passive nonlinear optical (visible, microwave, etc.) isolator (such as a diode). It may have potential applications in the field of optical detection (spectrometers), lasers and communications (circulators, wavelength division multiplexing circuits, etc.), etc. Another potential interest of these diodes is the fabrication of integrated laser gyroscopes for applications in aviation, etc.
Many applications, in particular in spectroscopy, laser technology and optical communications, require asymmetric or non-reciprocal optical elements. The most commonly used devices are optical isolators (for non-reciprocal transmission) and surface blazed diffractive gratings (for non-reciprocal reflection or diffraction). The optical isolators are very expensive, while the fabrication of blazed surface structure becomes more and more difficult (due to the increasing cost) as far as smaller features are required.
Bragg gratings (or reflectors) actually attract important attention in view of applications in Photonics and communication systems, in particularly, as filters in Wavelength Division Multiplexing (WDM) systems. In particular, the special Issue of the IEEE/OSA journal of Lightwave Technology on Applications of Photosensitivity and Quadratic Nonlinearity in Glasses, Vol. 15, No 8, (August 97) discusses such applications. The operation of these filters is based on periodic modulation of the refractive index n of used optical medium (fiber, guide, etc.) at a given wavelength (for example, the wavelength of communication xcexc). Indeed, the following conditions are required for its operation:
1) in the medium there exists a periodic (with periodicity xcex9) modulation of refractive index n(xcexc).
2) the xcexcis near or multiple of 2xcex9 (xcex9≈m xcexc/2, m=1, 2, 3 . . . xe2x80x94the Bragg resonance condition).
Usually, during fabrication, only the n(xcexc) of the medium is modulated periodically, while its absorption xcex1(xcexc) is relatively small and spatially uniform. Any possible (residual, etc.) modulation of xcex1(xcexc) in these cases is synchronized with the modulation of n(xcexc), that is, their relative spatial shift xcex94=0 (see FIG. 1). Such xe2x80x9ctraditionalxe2x80x9d Bragg filters have been shown to be excellent reflectors with very promising applications. However, in these elements, two counter propagating directions (+z and xe2x88x92z) are equivalent (reciprocal) and have the same diffractive characteristics. As can be seen in FIG. 1, the medium 20 spans from z=0 (30) to z=L (32). The period 24 is calculated between two maximum peaks and the relative spatial shift xcex9422, between a maximum peak of the refractive index curve 26 and a maximum peak of the absorption curve 28.
Non centro-symmetric photorefractive crystals have a non-reciprocal character as is discussed in P. Gxc3xcnter and J. P. Huignard, eds., Photorefractive Materials and Their Applications I-II, Vol. 61-62 (Berlin, Springer, 1989). That is, one can obtain non symmetrical energy exchange of counter propagating light beams, depending upon the orientation of the optical axis of the crystal (e.g., BaTiO3).
However, photorefractive materials are very costly, the formation of the shifted gratings (of n and xcex1) is slow and this effect is difficult to control in general, being a nonlinear phenomenon. That is why, there is no one practical application using photorefractive crystals.
The commonly used technique to fabricate an OI is the use of the following three elements in combination:
1) two polarizers (mutually oriented at 45 degrees),
2) a material possessing significant Faraday effect (constant of Verdet),
3) a source of relatively strong magnetic field.
Other devices have asymmetric reflection of signals (see FIGS. 3c and 3d). These are called diffractive elements. Most photons sent to the interface are either reflected or absorbed by the surface.
U.S. Pat. No. 5,559,825 to Scalora et al. discloses a new optical diode that permits unidirectional transmission of light. It comprises alternating layers of a low index material and a high index material.
U.S. Pat. No. 4,736,382 to O""Meara discloses an acousto-optical laser isolator for isolating unwanted, backwardly propagating laser beams. The isolator includes a first acousto-optical transducer Bragg cell to shift the frequency of a forwardly propagating beam by a first step, a second acousto-optical transducer Bragg cell to shift the frequency by a second opposite step and a filter between the two cells. The frequency shifting is effected by introducing phonon energy and requires the use of active devices.
Accordingly, a first object of the present invention is to provide several new techniques for easy and inexpensive fabrication of a new kind of non-reciprocal reflective/diffractive element.
Accordingly, another object of the present invention is a significant reduction of the noise level in optical communication systems (circulators, wavelength division multiplexing/WDM circuits, etc.), or any other optical systems using laser sources.
Another object of the present invention is to allow the direct fabrication of the discussed element in commercially available fibers, waveguides and other (including integrated) photonic circuits.
According to a first aspect of the present invention, there is provided a non-reciprocal optical element comprising:
an input port;
an output port;
a medium, optically coupled between said ports having two Bragg gratings recorded therein, said gratings being spaced apart, having a same period and each introducing different phase delays in photons reflected therefrom and being arranged so as to cause substantially reflection in light of a predetermined wavelength traveling from said input port to said output port and substantially absence of transmission in said light traveling from said input port to said output port.
Preferably, one of said gratings comprises changes in an absorption or gain of said medium and another of said gratings comprises changes in a refractive index in said medium.
According to a second aspect of the present invention, there is provided a non-reciprocal optical element comprising:
an input port;
a first frequency self-shifting optical element in optical communication with said input port;
an output port;
a second frequency self-shifting optical element in optical communication with said output port, said first and said second frequency shifting optical elements shifting a frequency of light in opposite directions and substantially by a same amount;
a filtering (e.g. Bragg grating) optical element in optical communication between said first and said second frequency self-shifting optical elements, wherein light of a predetermined wavelength traveling from said input port to said output port is shifted in frequency so as not to be reflected by said filtering optical element (e.g. Bragg grating), and said light traveling from said output port to said input port is shifted in frequency so as to be reflected or absorbed by said filtering optical element (e.g. Bragg grating).
Preferably, said first and said second frequency self-shifting optical elements comprise nonlinear and/or strongly dispersive materials.